Harmful materials found in many waste streams can present a significant risk to the environment and/or human health if left untreated. Government regulations often mandate that various organic, inorganic, chemical, and microbial components of a waste stream must be treated before the waste stream can be discharged to the environment. Examples of such waste streams include industrial and municipal wastewater, chemical processing effluent, animal waste, paper mill effluent, and others.
FIG. 1 presents a schematic of a treatment facility illustrative of prior art systems for treating municipal wastewater. With reference to FIG. 1, wastewater influent 100 enters a primary clarifier 102 of the wastewater treatment facility where raw sludge 104 is separated from the wastewater via flocculation, sedimentation, and other primary settling techniques. The wastewater from the primary clarifier 102 is then transported to an aeration basin 106, in which aerobic microorganisms 108 help treat the wastewater in the presence of air 110 that is pumped into the aeration basin 106. The wastewater is then transferred to a secondary clarifier 112, in which further settling can occur. Secondary sludge 114 is also collected in the secondary clarifier, and the treated wastewater 116 is transported to an effluent body of water or other distribution source (sometimes only after being subjected to certain advanced treatment procedures such as disinfection, for example).
Some of the secondary sludge 114 is recycled 118 back into the aeration basin 106 to help perpetuate the aerobic biodegradation process. The remaining secondary sludge 114 from the secondary clarifier 112 and the raw sludge 104 from the primary clarifier 102 are transported via a digester feed 120 to an anaerobic digester 122. In the anaerobic digester 122, anaerobic microorganisms 124 further degrade the sludge and yield by-product gases 126, such as methane 128. The digested sludge from the anaerobic digester 122 is transferred off as liquid 130 which can be applied directly to agricultural land or can be sent to a dewatering process 132. From the dewatering process, the centrate or liquid fraction 134 is typically returned to the front end of the treatment plant 100 for treatment, and the dewatered sludge cake 136 can be used for incineration, land application, or other appropriate uses.
Biosolids are a mixture of water and very small solid particles (primarily bacteria and protozoa). The particles carry a net negative charge that resists flocculation, i.e., the agglomeration of smaller particles into larger composite particles. In order to produce a matrix that has sufficient porosity to allow the movement of water through the matrix and accomplish dewatering, an organic polyelectrolyte or polymer is typically added to biosolids as a conditioning aid. In simple terms, these polymers are essentially long organic strands or ribbons with many active sites that have a net positive charge. The polymer neutralizes the negative charge on the biosolids and binds a particle to each active site on the polymer. The polymers have a tendency to stick to each other with the net effect that larger particles are created and the result is a porous matrix through which water can drain during the dewatering process.
Traditional polymer conditioning has a number of problems associated with it. Polymers are expensive and can account for a significant portion of the operating cost of a biosolids processing budget. The characteristics of biosolids change on a continual basis and it is difficult to provide the exact amount of polymer required at any given time to ensure that adequate performance is achieved without overdosing and wasting the polymer. The polymers normally used for conditioning biosolids are high molecular weight, high charge density materials, which are resistant to biological degradation and are known to be toxic to aquatic organisms. There is concern that any overdose of polymer could result in a release of the excess into the environment with toxic effects on fish and other aquatic species. The breakdown of polymers in stored biosolids has also been associated with the production of significant odors.
Waste streams may also be treated by incineration. Incineration is one of the few technologies that offers a technically sound solution for the complete destruction of pathogens, viruses, and toxic organics. In addition, it has the capability to process biosolids that contain high levels of contaminants, and which are, therefore, unacceptable for agricultural utilization. However, the high capital cost of a conventional incinerator and the extensive pre-processing (e.g., dewatering) of the biosolids make the process very expensive. Thus, incineration has typically been economical only in very large wastewater treatment facilities, in which it is possible to benefit from economy of scale.
In order to avoid the consumption of prohibitively expensive quantities of imported energy during the combustion process of prior art incineration systems, the biosolids must be dewatered to a cake with approximately 30% solids. At this level of dryness, the cake is a plastic, sticky, semi-solid material that can present significant operational problems related to the transfer of the cake from the dewatering device to the incinerator. Variations in the solids content of the cake and attendant variations in heat demand can also make the combustion process very difficult to control.
Stabilization of biosolids is a mandatory prerequisite for agricultural utilization of waste sludge and, depending upon the specific location, may be regulated by federal, state, or provincial authorities. The regulations protect human health and the environment from potential risks associated with pathogens contained in the biosolids. Regulatory bodies typically stipulate which treatment processes are acceptable and/or what levels of specific pathogens are allowable in the treated product. Conventionally, stabilization occurs through one or more of the following processes: biological degradation of organic matter, elevated pH, reduction of moisture, and waste handling according to specific time/temperature regimes. All of these processes are relatively expensive, and may be energy intensive, require large volumes of admixtures, or be difficult to control.
The electricity needed to operate conventional wastewater treatment facilities has traditionally been purchased from a distribution grid. There has been little incentive to generate electricity on site because power from the grid has been cheap and secure, the capital cost for generating equipment is high, and the only cheap source of fuel for power production is digester gas. However, there are a number of problems with utilizing digester gas as an energy source. Although it contains a high percentage of methane, digester gas is also saturated with water and contains significant quantities of hydrogen sulfide. This makes digester gas extremely corrosive, and extensive cleaning is required prior to its use. Furthermore, digester gas is only the byproduct of a waste treatment process, as opposed to being a production process in itself. The amount of gas produced is a function of the biosolids stabilization process and cannot be modified to meet changing demands. The combustion of methane-rich biogas also generates significant greenhouse gas emissions.
After most solids have been separated from influent wastewater, the remaining effluent is typically disinfected in preparation for its release into the environment. The disinfection of wastewater effluent has been historically accomplished through the addition of chlorine compounds. There are major health and safety concerns associated with handling chlorine compounds. In recent years, there have been increased concerns that chlorine can combine with organic material in the effluent to produce chlorinated organics, which are both toxic and potentially carcinogenic. Although some efforts are being made to substitute less toxic chlorine compounds, there is an industry-wide trend towards phasing out the use of chlorine as a disinfectant agent.
Other disinfection technologies employed in wastewater treatment involve the use of ultra-violet (UV) light or ozone to destroy the pathogens. Neither of these processes leaves any appreciable residual in the treated effluent to impact on the environment. However, the capital and operating costs of both of these systems are relatively high. In the case of a UV process, the capital costs include the construction of the flow-through mechanism, and the multiple UV bulbs (lamps) that are required. The operating costs include power, the replacement of bulbs, and regular cleaning of the bulbs. The major costs for disinfecting with ozone include the ozone generator and the commercial oxygen, which is used as the feed source. When air is used as the feed source, the size of the ozone generator must be approximately doubled, therefore doubling the capital cost.
Many of the uses for water in a waste treatment facility require relatively high quality water that is free of suspended particulate matter, but there is no need for it to be chlorinated or to have a chlorine residual present, as is the case for potable water. Final effluent does not meet these criteria due to the presence of suspended solids. It is therefore unsuitable as an alternative for applications such as making up polymer, because the suspended solids generate a polymer demand in the makeup water itself. Currently, waste treatment plants have no alternative for these higher quality end uses other than the use of potable water, which they must purchase from the municipal water supply system.
The present inventors have recognized a need for improved methods and systems for flocculation, incineration, dewatering, energy efficiency, stabilization, effluent disinfection, and production of high quality process water in a wastewater treatment plant and for treating other kinds of waste streams.